[ASan] Don't crash in DescribeHeapAddress if we don't know the current thread's ID

Also make DescribeThread easier to use.
This was firing on Dr.ASan runs, not sure how to repro InvalidTID in a simple test.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@190392 91177308-0d34-0410-b5e6-96231b3b80d8